I resolved snafu's situation with a Swiss mountain drive crankset,
made by Schlumpf, in Switzerland, I bought mine thru CM Wasson in Palo Alto Cal

another solution, My first purchase from him, was a FD adapter, [ala SP in UK]
to shift 2 chainrings, from the handlebars..
[i'm about to return it for credit to my card, unused, if any one wants it,

Thor has a FD kit for Dahon's too. instead.. ]

Schlumpf developed the mountain drive crank,
to turn utilitarian 3 speed bikes into 6 speed mountain pass hunters.
with another planetary gear .. instant shift at any speed even zero.

torque transfer is via a knob laid on the Brompton BB
between the Rear pivot and the BB, ...
quite tidy..


Oh, and I read, Brompton dealer in Scotland , Kinetics, does SA 8 speed hub kits ,
installed in the bikes if you wish..